A basic background description of such transmitters is to be found in a paper by C. J. Edgcombe and C. N. O'Loughlin entitled "The Television Performance of the Klystron Amplifier" published in The Radio and Electronic Engineer, September 1971. That paper describes in particular the 4-cavity klystron and the fact that it has a non-linear behaviour. As the input power is increased from zero towards the saturation value, with a single-frequency carrier wave input, the ratio of output power to input power decreases. A phase shift can also be introduced. This non-linearity is compensated by pre-correction either at video (baseband) frequency or more often at intermediate frequency (IF).
Such non-linearity exists in general when the change in level of the output signal is not exactly proportional to the change in level of the input signal, i.e., the input/output transfer characteristic is not precisely linear. All active devices are non-linear to some extent. Their non-linear effects tend to worsen with increases in output signal level and in conversion efficiency i.e., AC power out/DC power in. The effect, in the region of non-linearity of a periodic device that is incapable of harmonic response, is to produce output distortion of a single pure input tone which takes the form of amplitude change (normally a reduction) of the pure output tone, as noted above. Additionally, intermodulation products are produced when the input consists of more than one pure tone or is a complex signal. Finally variations in amplitude or phase of one of the tones are inversely superimposed on the others; this effect is termed cross-talk.
Intermodulation produces (IPs) are usually unwanted distortion components of sum and difference terms of the various signal frequencies Of a multi-tone signal or a complex waveform that is being processed by a non-linear device. These unwanted components from a complex waveform, such as a television signal, occur across a very broad band but are tailored by the response of a periodic device when set up to the normal pass band such as for a TV signal (normally from -1.25 MHz to +5.5 MHz relative to vision carrier frequency).
In-band intermodulation products, due to the TV signal components, are rarely at a high enough level to be a subjective nuisance particularly as they are non-coherent in their nature. However, in-band components created by the introduction of a relatively high level additional sound carrier, in common colour vision and sound amplifier working, can produce coherent and significantly large distortion tones at, typically, plus and minus 1.5 MHz relative to vision carrier frequency if the amplifier is sufficiently non-linear. This frequency of 1.57 MHz is the difference between the sound subcarrier, 6 MHz, and the vision subcarrier, 4.43 MHz. If this is an output-modulated amplifier such inter-modulation effects restrict the power output and efficiency obtainable from such a device for a given specified performance for unwanted spurious emissions in the image side band.
Likewise intermodulation components can be produced by the complex TV signal alone in the lower or image sideband of the TV channel. These are specified to be less than -37 dBp (relative to peak sync., carrier) except for image subcarrier at -4.43 MHz which is restricted to -47 dBp. These specified limits are to minimise lower adjacent channel interference in the adjacent service areas of other transmitters. This also restricts the efficiency to which a vision output power amplifier can be operated.
The presence of the image sideband components, even at the currently specified levels, also prevents the use of this part of a TV channel for other transmissions which in themselves would not be allowed to cause lower adjacent channel interference. It has been determined empirically that digital bearer carriers at a level of 43 dBp at -3.6 MHz and -5.6 MHz relative to vision carrier would not cause lower adjacent channel problems. With the above image sideband specification it will be necessary to further attenuate the radiated vision image interference components by about 15 db, even if a separate transmitter and aerial system were used for the digital bearers, in the lower sideband of an existing television transmission from the same site.
Thus, in summary, non-linearity restricts the power output and efficiency of an active device. This is important because of running costs with high power RF amplification stages. Non-linearity also prevents the effective utilisation of the lower sideband of TV channels due to the relatively high levels of regenerated image components. The in-band intermodulation products severely restrict the power output and the efficiency of devices such as klystrons operating as common colour vision and sound amplifiers.